The padded hip belt is compatible with any of our rucks with side molle. This excludes the Bullet and the GR3 (the GR3 has it's own hip belt that is only compatible with the GR3).
You can attach and detach the Padded Hip Belt through the Molle System. You just need to pull out the molle strap and below is a screenshot for reference as to where to find that. It may seem sewn in but make sure to check both sides of the attachment.
Also, here is a good link to a video to follow on how to do so: https://www.youtube.com/watch?v=Qlmn_HISABc